This invention relates to an ultrasonic cleaning apparatus, and particularly to an ultrasonic cleaning apparatus ideal for use in a semiconductor wafer cleaning process.
Ultrasonic cleaning is used as a most powerful physical cleaning method for the surface cleaning of various members and components. In particular, when ultrasonic cleaning is used for a wafer surface cleaning step in a semiconductor manufacturing process, whereas cavities readily forming at frequencies of 20 kHz to 50 kHz were used in the past, along with the microminiaturization of interconnection patterns damage to patterns caused by strong cavities came to be seen as a problem, and at present megasonic frequencies of 800 kHz to 1000 kHz with which it is possible to remove minute particles by means of acceleration of vibration without damaging patterns are being widely used. The cleaning effect of ultrasonic cleaning is raised when it is used together with chemical cleaning using for example an acid or an alkali.
FIG. 1 shows an ultrasonic cleaning apparatus of the related art. In this ultrasonic cleaning apparatus 1, a semiconductor wafer 7 (hereinafter called simply a wafer) constituting a thin plate member to be cleaned is immersed in a cleaning tank 5 filled with a cleaning liquid 6, and ultrasonic waves are radiated over the wafer 7 from an ultrasonic wave generator 3 mounted on one side wall of an ultrasonic wave tank 2 outside the cleaning tank 5. Cooling water 4 is passed through the ultrasonic wave tank 2 to prevent heating of the ultrasonic wave generator 3. The wafer 7 is held in a fixed attitude in the cleaning liquid 6 by being supported by supporting members 8a, 8b in which are formed grooves into which fits the periphery of the wafer 7. A number of such wafers 7 are held by the supporting members 8a, 8b in a stack perpendicular to the plane of the paper of FIG. 1, and the ultrasonic waves are radiated horizontally in parallel with the surfaces (cleaning faces) 7a of the wafers 7. The ultrasonic wave tank 2 and the cleaning tank 5 are both made of resin, and particularly for the cleaning tank 5 a fluorine resin having good resistance to chemicals is used.
FIG. 2 and FIG. 3 show other ultrasonic cleaning apparatuses of the related art. Parts in these figures equivalent to parts in FIG. 1 have been given the same reference numerals and will not be described in detail here. In the ultrasonic cleaning apparatus 11 shown in FIG. 2, the ultrasonic wave generator 3 is mounted on the bottom wall of the ultrasonic wave tank 2 and ultrasonic waves are radiated at the wafers 7 above from there. In the ultrasonic cleaning apparatus 12 shown in FIG. 3, multiple ultrasonic cleaning apparatuses 3a, 3b, 3c, 3d, 3e having different irradiation regions are mounted in two opposite side walls of the ultrasonic wave tank 2 and ultrasonic waves are radiated over the wafers 7 from both sides.
In these ultrasonic cleaning apparatuses 1, 11 and 12, due to the presence of the supporting members 8a, 8b holding the wafers 7 in the cleaning liquid 6, there is a shadow where ultrasonic waves, which travel highly rectilinearly, cannot reach. (This phenomenon is discussed in `Ultrasonic Engineering`, Masanori Shimakawa, Kogyo Chousa Kai (The Industrial Survey Association), 1975, pages 17 and 18.) That is, in the ultrasonic cleaning apparatuses 1 and 12 shown in FIG. 1 and FIG. 3 the supporting member 8b obstructs the advance of ultrasonic waves and creates an ultrasonic wave shadow region (shown with double hatching in the figures; similarly hereinafter) 9 in the cleaning faces of the wafers 7, and in the ultrasonic cleaning apparatus 11 shown in FIG. 2 the supporting members 8a and 8b create shadow regions 9a and 9b, and thus in all three of these cases there is the problem that the cleaning effect of the apparatus is reduced in these shadow regions. Although it is possible to reduce the sizes of these ultrasonic wave shadow regions 9, 9a and 9b by making the supporting members 8a, 8b thin and disposing them as near as possible to the lower end or the sides of the wafer 7 to solve this problem, there is clearly a limit to the improvement that can be achieved in this way.